Increasing demand for data communications has created a need for low cost half-duplex transceivers (radios) which operate in the 928/952 MHz frequency bands. In such transceivers, radios are designed such that the offset between the receiver and transmitter frequencies is fixed, such that the transmitter frequency is used as the first intermediate frequency (IF) local oscillator signal for the first mixer of the receiver, which operates as a super heterodyne receiver.
A block diagram of such a receiver is shown in FIG. 1. It may be seen that a reference signal from a reference oscillator 1 is applied to a 928 MHz signal synthesizer 3, which outputs its signal through a loop filter 5 to a voltage controlled oscillator, the output of which is passed into a power splitter 9. A portion of the signal is fed back to the synthesizer 3 to form a phase locked loop with loop filter 5 and voltage controlled oscillator 7, a portion is applied to an output power amplifier 11, and a portion, on line 13, is used as a local oscillator signal for the receiver. The signal portion that passes through power amplifier 11 passes through 928 MHz transmit filter 15, and antenna switch 17 to the antenna.
The 952 MHz signal received by the receiver is passed from the antenna through 952 MHz dielectric filter 19, low noise amplifier 21 and 952 MHz SAW filter 23 to the input of a first mixer 24. In mixer 24, the filtered 952 MHz signal is mixed with the 928 MHz signal from the transmitter, and the difference, a 24 MHz signal, is filtered in a 4-pole 24 MHz crystal filter 25.
The output signal from filter 25 is applied to one input to a second mixer 27, which receives a 23,545 MHz local oscillator signal from crystal controlled oscillator 29. The output of mixer 27 is at a standard IF frequency of 455 KHz., which is filtered in 455 KHz ceramic filter 31. The output signal from filter 31 is limited and discriminated by limiter 33 and discriminator 35. The resulting signal is passed through an audio filter 37 and a data comparator 39 to a data output line.
Thus it may be seen that with the transmit frequency 928 MHz and the receive frequency 952 MHz, each channel pair is arranged such that the offset between the transmit and receive frequencies is 24 MHz. The first local oscillator signal for the receiver is the same frequency as that of the transmitter, resulting in a first IF of 24 MHz. The modulation input to the synthesizer is zero during receive.
A benefit of this architecture is that only one synthesizer is required and since no signal source settling time is required for switching in either transmit or receive mode, the switching time can be fast.
However, this architecture has the disadvantage that the first IF frequency is at a non-standard IF frequency, and noise signals can be heterodyned into the second IF frequency. For that reason, a sharp cutoff very expensive IF filter is required, to restrict the first IF signal that enters the second mixer to only the 24 MHz IF signal, and to block any noise signals. The IF filter is typically a custom produced 4-pole crystal filter with a filter frequency equal to the transmit-receive signal offset.
The second IF filter 31 is typically a standard and inexpensive 455 KHz ceramic filter.